The invention relates to a method for coating a sheet-form textile material with an aqueous polyurethane urea dispersion. The invention relates further to the use of an aqueous polyurethane urea dispersion in the coating of a sheet-form textile material.
It has for a long time been part of the prior art to coat textiles, such as woven fabrics, knitted fabrics and nonwovens, with solutions of polyurethanes by the direct or reverse process. The articles obtained thereby are used, for example, in the manufacture of outer clothing, upholstery, bags, shoe uppers, tarpaulins, awnings and many other products.
Because of their particular properties, such as resilience and low-temperature flexibility, polyurethanes play an important part in the coating of sheet-form textile materials. Owing to increasing ecological demands, aqueous polyurethane dispersions are nowadays increasingly being used in order to reduce solvent emission. An overview of the various types and preparation processes is to be found in Houben-Weyl: “Methoden der organischen Chemie, Vol. E20, pages 1659 to 1692” or in “Ullmann's Encyclopaedia of Industrial Chemistry” (1992), Vol. A21, pages 667-682. Hereinbelow, the term polyurethane dispersions also refers to those dispersions that also contain urea groups in addition to the urethane groups.
A multilayer structure is very often used in the coating of textiles in order to impart the desired properties to the product. At the same time, each of the individual layers fulfils a particular function. The coating often consists of at least two layers, which are generally also referred to as coats. The uppermost layer facing the air is referred to as the top coat and is to render the surface as robust as possible against chemical and physical influences from the outside. These include, for example, properties such as resistance to hydrolysis and solvents, scratch resistance, a velvety or smooth surface, etc. The lowermost layer, facing the substrate, which joins the top coat to the textile, is also referred to as the adhesive coat. This layer must exhibit good adhesion on the one hand to the textile and on the other hand to the adhesive coat. A high degree of wet adhesion of the top coat is required, inter alia, because textiles can frequently come into contact with water.
Hitherto, polyurethane solutions have conventionally been used in the prior art as the adhesive coat. Such polyurethanes must not contain any internal and/or external hydrophilic structural units, which increases their wet adhesion. However, the use of organic solvents is increasingly undesirable for economic and, in particular, also ecological reasons.
In contrast thereto, polyurethanes that are used in aqueous polyurethane dispersions conventionally contain hydrophilic structural units in order to ensure that the polyurethane is dispersible in water. The hydrophilic structural units can be ionic and/or non-ionic. In the case of adhesive coats, however, the use of such polyurethanes has not been successful because the hydrophilic structural units lead to swelling of the polyurethane in water, and the adhesion of the coating is accordingly reduced. In the worst case, this can result in delamination.
Accordingly, it would be desirable to obtain polyurethanes that can be used in aqueous dispersions and nevertheless exhibit a high degree of adhesion.
From U.S. Pat. No. 6,713,131 B2 there is known a process for coating a substrate in which aqueous poly(isobutylene-co-paramethylstyrene) emulsions which have been functionalised by dimethyloxysilyl groups are applied to a polyamide together with other non-functional polyurethane dispersions. In order to emulsify the silylated polyurethanes in water, they contain external emulsifiers, which bring about a reduction in the wet adhesion. As discussed above, the presence of the external emulsifiers as hydrophilic structural units causes swellability of the polyurethane, which impairs the wet adhesion. This is counteracted by the polyurethanes containing self-crosslinking components that are reactive with the silyl groups, which improve the wet adhesion. However, silylated polyurethanes have the disadvantage that they are cost-intensive components. In addition, so-called VOCs (volatile organic compounds) are released when they are used—which is known to be undesirable from ecological points of view.
WO 99152676 A1 describes sulfone-group-containing polyester urethanes having terminal silyl groups. The polyester urethanes likewise contain hydrophilic groups, have been silyl-functionalised and are used in the form of an aqueous dispersion. They are used, for example, as an adhesive coat for abrasive belts. The polyurethanes described here are also functional reagents that crosslink on drying, whose swellability is compensated for in that manner. These compounds again have the disadvantage of a high price and poor environmental compatibility.
The object of the present invention is to provide a method for coating sheet-form textile materials in which polyurethanes are used in the form of an aqueous dispersion and with which there are obtained coated sheet-form textile materials which nevertheless exhibit high wet adhesion. In addition, the polyurethanes used are to be inexpensive to prepare and environmentally harmless.